HTML Guide
A <textarea> tag can’t be used inside an <a> tag.
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Last reviewed: July 19, 2023
Related W3C validator issues
An </a> end tag has been found to violate nesting rules. <a> tags can’t include other <a> tags inside. Most probable cause is an unclosed <a> tag, like in this example:
<a href="one.html">Page 1
<a href="two.html">Page 2</a>An <a> tag can’t include other <a> tags inside. Most probable cause is an unclosed <a> tag, like in this example:
<a href="one.html">Page 1
<a href="two.html">Page 2</a>The <iframe> HTML element represents a nested browsing context, embedding another HTML page into the current one.
As the iframe is a container that holds an embedded HTML page, it cannot be nested inside an a tag.
An <input> tag can’t be used inside an <a> tag.
A <label> tag can’t be used inside an <a> tag. Consider using other tags like <span>.
A button element, or an element with the role=button attribute, is not allowed to be nested inside an <a> element.
An <a> element cannot contain a descendant element with the attribute tabindex.
The <textarea> element does not have a type attribute.
The HTML <textarea> element represents a multi-line plain-text editing control, and is useful when you want to allow users to enter a sizeable amount of free-form text, for example a comment on a review or feedback form.
Spaces are not permitted in the href value for phone links; the phone number must be a continuous string without spaces or slashes.
The href attribute of an anchor (<a>) element defines the link’s destination. For phone numbers, the proper URI scheme is tel:, not callto:. According to the HTML standard and the WHATWG Living Standard, the phone number should contain only digits and may use plus (+) or hyphen (-) characters for formatting, but it should not include spaces or slashes.
Incorrect HTML:
<a href="callto:07142/ 12 34 5">Call us</a>Correct HTML:
<a href="tel:0714212345">Call us</a>With country code and optional formatting:
<a href="tel:+49714212345">Call us</a>For best compatibility and validation, always use the tel: scheme and ensure the phone number string contains only allowed characters.
The validator error occurs when an element such as an a, button, or custom widget includes aria-controls="" (empty) or whitespace-only. The aria-controls attribute takes one or more space-separated id values (IDREFS). Each referenced id must exist exactly once in the same document. Leaving it empty violates the ARIA and HTML requirements and provides no usable relationship for assistive technologies.
Why this matters:
- Accessibility: Screen readers rely on aria-controls to announce relationships between controls and controlled regions (e.g., a toggle and its panel). An empty value misleads AT or adds noise.
- Standards compliance: HTML and ARIA require at least one non-whitespace id. Empty values cause validation failures.
- Robustness: Incorrect references can confuse scripts and future maintainers, and break behavior when IDs change.
How to fix it:
- Only add aria-controls when the element truly controls another region (show/hide, sort, update).
- Ensure the controlled element has a unique id.
- Set aria-controls to that id (or multiple space-separated IDs).
- Keep the reference in sync if IDs change.
- If nothing is controlled, remove aria-controls entirely.
Examples
Invalid: empty aria-controls (triggers the error)
<a href="#" aria-controls="">Toggle details</a>Valid: control a single region
<div id="details-panel" hidden>
Some details...
</div>
<a href="#details-panel" aria-controls="details-panel">Toggle details</a>Valid: control multiple regions (space-separated IDs)
<section id="filters" hidden>...</section>
<section id="results" hidden>...</section>
<button type="button" aria-controls="filters results">Show filters and results</button>Valid: remove when not needed
<a href="#">Toggle details</a>Minimal complete document with proper usage
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>aria-controls Example</title>
</head>
<body>
<button type="button" aria-controls="info" aria-expanded="false">Toggle info</button>
<div id="info" hidden>
Extra information.
</div>
<script>
const btn = document.querySelector('button');
const panel = document.getElementById(btn.getAttribute('aria-controls'));
btn.addEventListener('click', () => {
const expanded = btn.getAttribute('aria-expanded') === 'true';
btn.setAttribute('aria-expanded', String(!expanded));
panel.hidden = expanded;
});
</script>
</body>
</html>Tips:
- Use aria-controls for functional relationships (control affects content), not just visual proximity.
- Combine with aria-expanded when toggling visibility to convey state.
- Verify that every id in aria-controls exists and is unique; avoid dynamic mismatches created by templating or component reuse.